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<H2>3.8 Pipelines</H2>

<DL class="page-index">
  <DT><A href="#overview">Overview</A></DT>
  <DT><A href="#examples">Examples</A></DT>
  <DT><A href="#headers">Headers</A></DT>
  <DT><A href="#reference">Reference</A></DT>
</DL>

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<A NAME="overview"></A>
<H2>Overview</H2>

<P>
    A pipeline is an expression of the form 
</P>
<PRE CLASS="broken_ie">filter<SUB>1</SUB> | ... | filter<SUB>n</SUB> | filter-or-device</PRE>
<P>
consiting of one or more filters and an optional device, joined using <CODE>operator|</CODE>. Pipelines are a convenient way to pass chains of Filters and Devices to the constructor or <CODE>push</CODE> function of a <A HREF="../classes/filtering_stream.html"><CODE>filtering_stream</CODE></A> or <A HREF="../classes/filtering_streambuf.html"><CODE>filtering_streambuf</CODE></A>. 
</P>

<P>
    In order for instances of a model of Filter to appear in a pipeline, it must be declared <A HREF="../concepts/pipable.html">Pipable</A> using the macro <A HREF="#boost_iostreams_pipable"><CODE>BOOST_IOSTREAMS_PIPABLE</CODE></A>.
</P>

<P>
   Pipelines for C++ filtering were introduced by Jan Christiaan van Winkel and John van Krieken. <I>See</I> <A CLASS="bib_ref" HREF="../bibliography.html#van_winkel">[van Winkel].</A> 
</P>

<A NAME="examples"></A>
<H2>Examples</H2>

<P>The following example defines a <A HREF="../concepts/pipable.html">Pipable</A> InputFilter.</P>

<PRE CLASS="broken_ie"><SPAN CLASS='preprocessor'>#include</SPAN> <A CLASS='header' HREF='../../../../boost/iostreams/concepts.hpp'><SPAN CLASS='literal'>&lt;boost/iostreams/concepts.hpp&gt;</SPAN></A>
<SPAN CLASS='preprocessor'>#include</SPAN> <A CLASS='header' HREF='../../../../boost/iostreams/pipeline.hpp'><SPAN CLASS='literal'>&lt;boost/iostreams/pipeline.hpp&gt;</SPAN></A>

<SPAN CLASS="keyword">namespace</SPAN> io = boost::iostreams;

<SPAN CLASS="keyword">class</SPAN> my_filter : <SPAN CLASS="keyword">public</SPAN> io::input_filter {
<SPAN CLASS="keyword">public:</SPAN>
    <SPAN CLASS='omitted'>...</SPAN>
    <SPAN CLASS="keyword">template</SPAN>&lt;<SPAN CLASS="keyword">typename</SPAN> Source&gt;
    <SPAN CLASS="keyword">int</SPAN> get(Source&amp; src)
    {
       <SPAN CLASS='omitted'> ...</SPAN>
    }
};
BOOST_IOSTREAMS_PIPABLE(my_filter, 0)
</PRE>

<P>No semicolon is required (or allowed) following the macro invocation. The following example shows a <A HREF="../classes/filtering_stream.html"><CODE>filtering_stream</CODE></A> constructed from a pipeline</CODE>.</P>

<PRE CLASS="broken_ie"><SPAN CLASS='preprocessor'>#include</SPAN> <A CLASS='header' HREF='../../../../boost/iostreams/filtering_stream.hpp'><SPAN CLASS='literal'>&lt;boost/iostreams/filtering_stream.hpp&gt;</SPAN></A>
<SPAN CLASS='preprocessor'>#include</SPAN> <A CLASS='header' HREF='../../../../boost/iostreams/device/file.hpp'><SPAN CLASS='literal'>&lt;boost/iostreams/device/file.hpp&gt;</SPAN></A>
<SPAN CLASS='preprocessor'>#include</SPAN> <A CLASS='header' HREF='../../../../boost/iostreams/filter/counter.hpp'><SPAN CLASS='literal'>&lt;boost/iostreams/filter/counter.hpp&gt;</SPAN></A>

<SPAN CLASS="keyword">namespace</SPAN> io = boost::iostreams;

<SPAN CLASS="keyword">int</SPAN> main()
{
    <SPAN CLASS='comment'>// Write to the file "hello," counting</SPAN>
    <SPAN CLASS='comment'>// the number of lines and characters</SPAN>
    io::filtering_ostream out(io::counter() | io::file("hello"));
    <SPAN CLASS='omitted'>...</SPAN>
}</PRE>

<A NAME="headers"></A>
<H2>Headers</H2>

<DL class="page-index">
  <DT><A CLASS="header" HREF="../../../../boost/iostreams/pipeline.hpp"><CODE>&lt;boost/iostreams/pipeline.hpp&gt;</CODE></A></DT>
</DL>

<A NAME="boost_iostreams_pipable"></A>
<A NAME="reference"></A>
<H2>Reference</H2>

<PRE CLASS="broken_ie">

<SPAN CLASS='preprocessor'>#define</SPAN> BOOST_IOSTREAMS_PIPABLE(filter, arity) <SPAN CLASS='omitted'>...</SPAN>

</PRE>

<H4>Description</H4>

<P>Defines the overloads of <CODE>operator|</CODE> necessary for a <A HREF="../concepts/filter.html">Filter</A> to appear in pipelines.</P>

<A NAME="macro_params"></A>
<H4>Macro parameters</H4>

<TABLE STYLE="margin-left:2em" BORDER=0 CELLPADDING=2>
<TR>
    <TR>
        <TD VALIGN="top"><I>filter</I></TD><TD WIDTH="2em" VALIGN="top">-</TD>
        <TD>The name of the <A HREF="../concepts/filter.html">Filter</A> to be declared <A HREF="../concepts/pipable.html">Pipable</A></TD>
    </TR>
    <TR>
        <TD VALIGN="top"><I>arity</I></TD><TD WIDTH="2em" VALIGN="top">-</TD>
        <TD>The <A HREF="../concepts/filter.html">Filter</A>'s template arity, <I>i.e.</I>, the number or its template parameters; a value of <CODE>0</CODE> indicates that it is not a class template</TD>
    </TR>
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